EMRFD Message Archive 5619

Message Date From Subject
5619 2010-12-31 08:22:54 klopfensteinrex Using Feedthru capacitors?
I am working on building the "Modular Direct Conversion Receiver" described in EMRFD pages 8.13-8.15. I plan to house the Universal VXO in its own box (scraps of PCB). I plan to run the output through some sort of RF connector. I also plan to run the +12VDC though a feedthru capacitor.

I have searched the internet for information on sizing feedthru capacitor for a specific application. Can anyone point me to a good reference?

Rex Klopfenstein
5623 2010-12-31 11:18:08 KK7B Re: Using Feedthru capacitors?
Someone might have a reference, but what you really need to know is how it works. A feedthu capacitor for a power supply connection into a metal box containing electronics forms a low pass filter with decoupling resistors and lead inductance, somewhere well below the frequencies in the box. It also serves as an RF short to the box wall at any frequencies inside the box. Those functions are usually enhanced by other components inside and outside the box, including series resistors and bypass capacitors on individual circuit blocks.

The short answer to your question is: they are not critical, and anything you have can be used by adding other components inside the box in series or parallel. Common sizes are 0.01 uF and 0.001 uF. I usually add series inductance and resistance (a ferrite bead slipped over a resistor lead) inside the box, 0.1uF bypass capacitors at each stage, and lots of series decoupling resistors. Study some of the photos in EMRFD to see how it's done. Anything that has gain at low frequencies (including MMICs) also gets an electrolytic between ground and the power supply near the active device. Calculate series resistor values using ohms law and a small amount of voltage drop at your expected current. 10 ohms and 100 ohms are common in my projects. Note that some oscillators take a lot more current to start. Then calculate the RC time constant with that resistor value and your feedthru cap. Feedthru caps are particularly effective at HF-VHF on modules that don't take much current.

A good ap note on bypassing and decoupling would be useful--but many of us who use feedthru capacitors learned how to use them by tearing apart military surplus gear and test equipment and thinking about what we encountered. Every situation is different, and there's no way around thinking about what you need.

Reverse engineering ham gear with feedthru caps is less valuable, as there may have been less thinking going on. Sometimes a feedthru cap was used just because it looked cool and provided a convenient place to attach the power supply wire. That's ok, but you can get better performance out of them by thinking a bit.

If you enjoy homebrewing with bits of brass, you can build good substitutes for DC feedthru caps using lengths of brass tube with a couple ferrite beads inside to hold a bare wire in the center, and short leaded capacitors soldered between the wire and brass tube inside and outside the box. Another variation uses a resistor inside the tube. Drill a hole in the circuit board material wall that just passes the brass tube, and solder it all the way around on both sides. Then solder the caps with zero lead length between the lip of the brass tube and the center wire.

Finally, for really good isolation, consider box-inside-box construction. Build the VFO in a sealed PC board material box. Outside the box, use an insulated shaft coupling and an insulated shaft to the outside world to tune the VFO. Put that whole assembly inside a second box. Run the shaft through a length of 1/4" id brass tubing an inch or so long, again soldered to both the inside and outside of the box. That makes a waveguide beyond cutoff. I've achieved greater than 120 dB isolation that way in a VFO that can be left running on frequency, and can't be heard on the stati
5625 2010-12-31 18:12:00 john lawson Re: Using Feedthru capacitors?
Hi Rick, I read with interest your mini tutorial on feedthru caps....After reading your post I headed off to the junk box and pulled out some feedthru caps of different values and measured the inductance and capacitance of each one to satisfy my curiosity.
Two measurements stood out, one of a 5000 pf feedthru cap pulled from military equipment sold by Max Gain. Measuring with my  AADE meter the inductance was .03 uH and capacitance of 4700 pf.  Other commercial feedthru's were also measured with inductance's from .01 to .02 uH.  Then came the other interesting measurement: The feedthru is a Pi Filter from Spectrum Control 1201-066 sold by Mouser for about $6.00. This is the feedthru that Ed, KG4ARN put me on to to correct the baseline rise problem in my QST Tracking Generator sometime ago....It measured 2.05 uH and 8300 pf.   It's attenuation is advertised at 68 dB at 100 MHz.  Finally I dug out some FB43-101 ferrite beads from Amidon. It's the ferrite bead that was used on the gate of the 40673's in the PR project some years ago. Sliding one on a wire lead gave an inductance of .58 uH.  Looks like one or more of these inside that brass tube with caps on either side to roll your own pi
filter would be an interesting experiment.  Thanks for taking the time to post your information. I for one found enlightening and helpful.  John K5IRK

5626 2010-12-31 19:12:40 KK7B Re: Using Feedthru capacitors?
Hi John,

Well, now we're even--I sure got a ton of good informati